Nanoarchitecture factors of solid electrolyte interphase formation via 3D nano-rheology microscopy and surface force-distance spectroscopy

The solid electrolyte interphase in rechargeable Li-ion batteries, its dynamics and, significantly, its nanoscale structure and composition, hold clues to high-performing and safe energy storage. Unfortunately, knowledge of solid electrolyte interphase formation is limited due to the lack of in situ nano-characterization tools for probing solid-liquid interfaces. Here, we link electrochemical atomic force microscopy, three-dimensional nano-rheology microscopy and surface force-distance spectroscopy, to study, in situ and operando, the dynamic formation of the solid electrolyte interphase starting from a few 0.1 nm thick electrical double layer to the full three-dimensional nanostructured solid electrolyte interphase on the typical graphite basal and edge planes in a Li-ion battery negative electrode. By probing the arrangement of solvent molecules and ions within the electric double layer and quantifying the three-dimensional mechanical property distribution of organic and inorganic components in the as-formed solid electrolyte interphase layer, we reveal the nanoarchitecture factors and atomistic picture of initial solid electrolyte interphase formation on graphite-based negative electrodes in strongly and weakly solvating electrolytes.

(2) the authors compared the solvation structure of EC:DMC and DX based electrolytes by observing the differences in Force Curves on electrode|electrolyte interface. This piece of information is very important in figuring out the SEI formation mechanism, but is totally downplayed in the title and the abstract of the paper. For one thing, the 3D-rheology microscopy does not deserve the current attention in the title.
(3) it will be highly interesting to use the methods reported here to study the difference between EC and PC in their interaction with graphite anode and the SEI formation. It is probably much more relevant to the battery field than DX on graphite. The authors are recommended to provide data and discussion on PC-based electrolytes.
Reviewer #2 (Remarks to the Author): SEI is a critical but complex topic in battery research, and it is still challenging to fully understand through both computational and experimental investigation. Developing any new methodology, therefore, is very important to the field. This work presents a new method using both electrochemical atomic force microscopy and 3D nano-rheology microscopy to identify the chemical composition of SEI and illustrate its early formation process. However, I feel that some experiments are not explained very clearly, and the MD simulation part doesn't provide much useful support or insight. Here are some detailed questions for the authors to consider: (1) On Page 3, please add which electrolyte systems to study in this work before the RESULTS section.
(2) Fig 1K, what is the peak near 160 nm? (3) Page 6, Please add a clear discussion to explain the distinct SEI structures between the edge and basal planes via XPS studies, since this was mentioned at the beginning of the next part. (4) Overall, I feel the figures are all too small. Some letters and features are hard to read. (5) Figure 4c is kind of misleading. The authors always mention the EC-Li coordination for ECbased electrolyte, which is also demonstrated in Figure 4c that only the EC coordination appears in the inner-most layers next to the anode surface, it should be noted that the real electrolyte used here is 1 M LiPF6 in EC/DMC (please also explain the molar or volume ratio between EC/DMC on page 4). It is expected that DMC also coordinates to Li, probably even at a larger number depending on the EC: DMC ratio, because the literature (Fundamental Research 1 (2021) 393-398) suggests a stronger Li-DMC binding energy than Li-EC. This should be able to validate through MD simulations. (6) The XPS in Figure S11 e and h also shows the Li-F SEI component on both basal and sectional planes, is this due to the decomposition of anion? The relative Li-F peak is higher on the basal plane, why? (7) Why the similar Figures in Figure S11 are not measured for the DX-based electrolyte? (8) Are insets figures in figures 4e and 4f from MD simulations? Li should also be highlighted. (9) It is mentioned on page 11 that "The DX solvent molecules rarely exist inside the inner Helmholtz layer" which is not supported by the inset in Fig 4f. (10) MD simulation methods are not described clearly which is not enough for others to reproduce the work. The MD results are also not reported and explained at all, not sure what is the purpose of the MD part. The MD results should provide detailed and clear information on the interfacial structure of the electrolyte. If the results were not discussed, this part should be removed.
We are grateful to the Reviewers for the in-depth comments and for the overall positive evaluation of the submitted manuscript. We carefully revised the manuscript and the SI full in response to the comments, with the point-by-point answers listed below. In particular, we feel that the manuscript now resolves the concerns regarding the invasiveness of the microscopy technique, interpretations and data presentation of the MD simulations, and includes more detailed discussions, thanks for the thoughtful and in-depth questions and comments. We also feel that the revised manuscript now even more clearly brings together concepts of direct observation of anode-electrolyte interface structure, from the initial atomic-scale EDL surface towards the resultant 3D nanostructures of SEI, to provide a better impact on a broader scientific community.
We highlighted all revisions in the "highlighted" version of the revised manuscript.

Reviewer #1 (Remarks to the Author):
The manuscript by Chen and Kolosov et.al. reports on a scanning probe-based investigation on the SEI structure, mechanical properties (modulus and viscosity), as well as salvation structure. The important information, is then the link between the solvation structure and the SEI structure, i.e., the possible SEI forming mechanism. This is an highly interesting work, and is recommended to be considered published in Nature communication once the following concerns are addressed: Response REV1: We sincerely thank Reviewer 1 for the positive comments on our work, the manuscript has been carefully amended according to the Reviewer's constructive suggestions, the point-by-point response is attached below.
(1) the so-called 3D nano-rheology microscopy is an intriguing technique, but it is invasive. With the nano probe diving into the SEI layer by hundreds of nanometers, the components in SEI is inevitably pushed around and severely disturbed. Furthermore, the measured mechanical properties, especially the viscosity, could be sensitively dependent on the loading speed of the normal pressure on the tip, as well as the shear force frequency and amplitude. It would be necessary for the authors to comments on these effects, especially in the context of this study, what effects do these factors have regarding the spatial resolution in-and out-of-plane, and how quantitatively reliable the mechanical property measurements are. REV1_R1: We gratefully acknowledge this useful comment on invasiveness. Indeed, 3D nano-rheology is an invasive nano-indentation method, while its invasiveness is the key to accessing the subsurface structure of ultrathin SEI layers, similar to the 3D tomography AFM 1 and XPS etch milling methodology. In other words, 3D nanorheology is a complementary methodology for the traditional electrochemical AFM 2, 3, 4 which is limited to merely studying the surface topography/properties of SEI layer. In this work, we used the traditional non-invasive operando AFM to observe the dynamic SEI formation, and after the SEI film is grown to the particular cycle point on the electrode surface. We then performed in-situ (in electrolyte environment) 3D nano-rheology on the as-yet undisturbed SEI, in which each image pixel is "fresh" with undamaged SEI nanostructures. Therefore, by taking advantage of invasive nano-indentation with shear modulation and high-lateral resolution of AFM technique, 3D nano-rheology can differentiate the elastic and viscous component distribution inside the SEI layers, which is not available in traditional statistic AFM force spectroscopy techniques 5,6,7,8 .
We thank the Reviewer's and Editors for suggestion to describe the 3D-NRM for the broader audience, and to clarify effects of the tip loading/modulation speed and mechanical property quantification and add the following in the main manuscript: Page 6, lines 3-7, "3D-NRM maps the nanomechanical viscoelastic properties of the SEI layer by penetrating it with the AFM tip. As the probing tip approaches the surface from the electrolyte, a sample is oscillated laterally at a small amplitude of few nm, and the in-phase and out-of-phase components of the lateral (shear) force acting on the tip is measured. By taking the derivative of the force over penetration increments, the effective shear modulus (G) and viscosity (η) distribution as a function of the depth of SEI layer is evaluated (SI, Fig. S2.1e)." Page 7, lines 5-9, starting with "3D-NRM, working at off-resonance mode, …" And the following paragraph Page 7, lines 10-17, "Due to very small dithering amplitude, relatively fast oscillation period (~1 ms) and slow measurement cycle (~1 s), the response of the 3D SEI has shown to be practically independent on the approach cycle frequency (varying from 0.1 to 10 Hz), while clearly dependent on the dithering frequency allowing to reliably evaluate the viscous (loss) and elastic (storage) component of the SEI viscoelastic response. Each vertical probing takes data particular area (roughly 20x20 nm2, with next measurement at the "safe" distance of about 50 nm apart, for thinner SEI the dimensions will be correspondingly smaller).
The probed area is disturbed and not probed again, while still providing a fully representative 3D snapshot of the particular state of the SEI (see detailed description in SI sections S1-S3 for both methodology, typical experimental results, and calibration)." We also add a detailed description of the dynamic behaviour of the 3D-NRM in the revised SI Page S7, lines 28-51, starting with "The effect of vertical loading …" Page S8, Figure S3.2 with the caption.
(2) the authors compared the solvation structure of EC:DMC and DX based electrolytes by observing the differences in Force Curves on electrode|electrolyte interface. This piece of information is very important in figuring out the SEI formation mechanism, but is totally downplayed in the title and the abstract of the paper. For one thing, the 3Drheology microscopy does not deserve the current attention in the title. REV1_R2: We are grateful for the good suggestion. We amended the title that now reads as "Nanoarchitecture factors of solid -electrolyte interphase formation via 3D nano-rheology microscopy and surface force-distance spectroscopy" and added the following sentence to the abstract Page 1, lines 22-25, "By probing solvent molecules -ions arrangement within the EDL and quantifying 3D mechanical property distribution of organic and inorganic components in the as formed SEI layer, we reveal the nanoarchitecture factors and atomistic picture of initial SEI formation on graphite anode in strongly and weakly solvating electrolytes." to highlight the links between the observed solvation structures and SEI formation mechanism.
(3) it will be highly interesting to use the methods reported here to study the difference between EC and PC in their interaction with graphite anode and the SEI formation. It is probably much more relevant to the battery field than DX on graphite. The authors are recommended to provide data and discussion on PC-based electrolytes.  Figure R1 below, in which the carbon atomic steps exfoliation ( Figure R1a) in the basal plane, as well as the damages to the HOPG model samples at the sectional plane ( Figure R1b) 9 , are the two main obstacles for performing further detailed high-resolution in-situ/operando electrochemical AFM measurements 10,11 . To further address this question, we added Section S5 and Figure S5.1 to the revised SI (see detail discussion below) and references to these data in the main manuscript in P9, lines 3-7. containing DEC co-solvent can also co-intercalate and slightly weak the Van Der Waals interaction of carbon layers, generating many nano-blisters/bubbles trapped inside graphite 12, 13 ( Figure S5.1b), which is detrimental for the mechanical property quantification of later formed SEI layers; In EC/DMC mixed electrolyte, the solvent cointercalation and sequential decomposition effectively sealed the carbon step edges 14 and form carbon edge wrappings at initial lithiation stage ( Figure S5.1c), preventing the further graphite exfoliations/delamination. EC/DMC passivated HOPG surface can thereby serve as an idea "solid-substrate" for the study of nanoscale mechanical properties of SEI layers. The detailed solvent/co-solvent co-intercalation behaviour will be discussed in our next work. Besides, since this study focuses on the effects of solvent co-intercalation and solvation structure on SEI formation, we therefore selected the weakly solvated solvent which has distinct solvation structures with lithium ions 15,16,17 , to eliminate the solvent co-intercalation resulted in carbon layer exfoliation and sample damages 18 .

Reviewer #2 (Remarks to the Author):
SEI is a critical but complex topic in battery research, and it is still challenging to fully understand through both computational and experimental investigation. Developing any new methodology, therefore, is very important to the field. This work presents a new method using both electrochemical atomic force microscopy and 3D nano-rheology microscopy to identify the chemical composition of SEI and illustrate its early formation process. However, I feel that some experiments are not explained very clearly, and the MD simulation part doesn't provide much useful support or insight. Here are some detailed questions for the authors to consider: (1) On Page 3, please add which electrolyte systems to study in this work before the RESULTS section.
Rev2_R1: We thank the Reviewer's question. The electrolyte systems are now specified before the RESULTS section and the cross-comparison of different electrolyte systems was further added in the revised manuscript.
Page 3, lines 32-34, "We use a matrix of two morphologically dissimilar but chemically identical surfaces of typical carbon electrode material (basal and edge graphene planes) and two solvent-electrolyte systems, polar solvent (Ethylene Carbonate, EC, mixed with Dimethyl Carbonate, DMC) and non-polar (1,4 Dioxane, DX) solvent, to get direct insight into the atomistic pictures for the underlying influence of solid-liquid interfacial nanostructure on the initial SEI formation." (2) Fig 1K, what is the peak near 160 nm? Rev2_R2: We thank Reviewer's question. The peak near 160 is one of the wrapping carbon steps after the solvent cointercalation/decomposition. We replotted the topography images in three-dimensional as shown in SI Section 4 Fig.   S4.5 (see below), the line structures observed on the sample surface during the charge/discharge cycles are the SEI formed and accumulated at the carbon step edge 14,19 . In other words, the peak neat 160 nm ( Figure 1j in the main text) are the electrolyte decomposition products preferentially accumulated at one of the step edges. To better explain this SEI decomposition-induced carbon step edge expansion/wrapping, we specify this section profiles in the legend and add the below 3D presented topography in the revised supplementary information.     Figure S11 e and h also shows the Li-F SEI component on both basal and sectional planes, is this due to the decomposition of anion? The relative Li-F peak is higher on the basal plane, why?
Rev2_R6: We gratefully acknowledge the comment. We believe the Li-F component is mainly from the hydrolysis and electrochemical decomposition of PF6anion 14,28 . In our graphite model sample, the main SEI components formed in the basal plane derive from the heterogeneous transfer of electrons directly to the salt anions and solvents. This heterogeneous transfer of electrons results in electrolyte decomposition precipitates 29 , including a large amount of anion decomposition products which contain more LiF. By contrast, the SEI layer formed on the edge plane mainly derives from the decomposition of co-intercalated EC solvents which results in the accumulation of lithium ethylene di-carbonate (LEDC), or lithium ethylene mono-carbonate (LEMC) 30 in the very end of intercalation active edge planes 29 . The accumulation of these organic species, instead of anion decomposition precipitations, forms the main SEI components on the sample sectional plane. Therefore, the sectional plane shows lower Li-F peak intensity compared to the basal plane. Figure S11 are not measured for the DX-based electrolyte?

(7) Why the similar Figures in
Rev2_R7: We thank the Reviewer for noticing this. The SEI formed on two planes in the DX-based electrolyte are both anion-derived SEI (independent of graphite crystal orientations) that has similar chemical components, and thereby were not discussed in detail. The detailed discussion and Li1s, S2p, N1s, O1s, C1s, and F1s high-resolution spectra of the basal plane and sectional plane SEI formed in DX electrolyte has been added to Rev2_R8 We acknowledge this comment from Reviewer. We highlighted the Lithium-ions in the insets in Figure 4e and 4f and add these two MD simulations results in the SI SI, Section 11, Figure S10.2.
(9) It is mentioned on page 11 that "The DX solvent molecules rarely exist inside the inner Helmholtz layer" which is not supported by the inset in Fig 4f. Rev2_R9: We thank reviewer's useful comment, the wrong statement has been changed to "due to the weak coordination effect between DX and lithium … FSIstill dominates the first solvation sheath inside the EDL" which is supported by the Raman and MD simulation in Figure S11

Reviewer #1 (Remarks to the Author):
The revision has satisfied my concerns in the previous review. I recommend the manuscript to be published without further revision.
Reviewer #2 (Remarks to the Author): I have a few more questions regarding the revised manuscript.
Regarding MD simulations: (1) How do you decide the simulation box size and number of electrolyte species? (2) There is no description to explain how the -1 V vs. PZC was simulated ( Figure S10.3).
(3) Are 3D periodic boundary conditions applied? According to Figure S10, it appears that a vacuum space is added to separate the two electrodes. If the simulation uses 3D periodic boundary conditions, the 5 nm vacuum space is too short for separating two oppositely charged electrodes. It is recommended to use the vacuum length of twice the electrolyte box to validate your results again.
Page 12, lines 319-326, here, the authors discuss the different existence of the PF6 and FSI anion in Li-solvation structures near the electrode due to different lithium solvent coordination abilities. However, the two electrolyte systems studied here have both different solvents (EC/DMC vs. DX) and lithium salts (LiPF6 vs. LiFSI). It is uncertain whether different anion chemistries can also lead to different interfacial chemistries in the same organic solvent electrolyte.
We thank the Reviewers for the positive evaluation of the manuscript and helpful comments. Please find attached the detailed response to the comments raised by the Reviewer 2.

Reviewer #1 (Remarks to the Author):
The revision has satisfied my concerns in the previous review. I recommend the manuscript to be published without further revision.
Response: We thank the Reviewer for the positive feedback on the revised manuscript.

Reviewer #2 (Remarks to the Author):
I have a few more questions regarding the revised manuscript.

Regarding MD simulations:
(1) How do you decide the simulation box size and number of electrolyte species?
Response: We thank the Reviewer for the in-depth and constructive comments on MD simulation and are answering the questions raised in details below.
The size of the simulation box and the number of electrolyte species were determined by ensuring the electrostatic crosstalk effect between two carbon electrodes can be eliminated, meanwhile, enough molecules can also be used for the statistical analysis, such as coordination radius distribution functions (RDF) and interfacial ion distribution functions.
To be more specific, first of all, a 10 nm distance between two electrodes was chosen such that the forces on the molecules in the middle of the simulation box were statistically the same as those in the bulk simulations without electrodes 1, 2 . Additionally, in every simulation result, we also examined the solvation structures of lithium-ions in the polarized bulk electrolyte by RDF ( Figure S10.3 b and S10.3 d of supplementary information, SI), which is similar to that of bulk electrolyte under the un-polarized state (Figure S11.1 c and S11.1 d of SI). Namely, the half distance of the two electrodes was controlled to be larger than the Debye length of EDL (around 2-3 nm according to the force curve experiments). Besides, the electrode area size of around 5.1×5.1 nm 2 was chosen to ensure the continuity of the in-plane periodic structure of carbon hexagonal rings in two electrodes.
Once the above simulation box size is determined, the number of each electrolyte species should be enough for the statistical analysis of EDL structures and lithium-ions' solvation structures. The detailed numbers of each species can be calculated according to the densities, concentration and volume ratio of solvents in the electrolyte as discussed below. We add the detailed statement in this in the revised SI (lines 34-41, Page 2  and lines 3-27, Page 3).
"The detailed numbers of each species can be calculated according to the densities, concentration and volume ratio of solvents in the electrolyte. The volume of simulation box is around 2.60 × 10-19 cm3. For the 1M concentration electrolytes, the simulation box contains about 2.60 × 10-22 mole salt molecules, which corresponds to about 156 LiPF6 or LiFSI. Since the electrolyte is dilute, we ignored the volume changes of the simulation box before and after adding salt. (This results on the error of less than 1% for these values, see the results in Table R1 below) The volume ratio of EC: DMC is 1:1, therefore taking the density of 1.33±0.01 g/cm 3 for EC and 1.07±0.01 g/cm3 for DMC at room temperature, the mass in the simulation box will be around for EC 1.72-1.74 × 10-19 g and for DMC 1.38-1.40 × 10-19 g, corresponding to the numbers of around 1175-1189 and 920-936 for EC and DMC molecules, respectively. The molecule numbers within this range are in an acceptable error range. Similarly, the number of DX in the simulation box and is around 1638, by using a room temperature density value of 1.03 g/cm 3 .
For a more precise estimation using the final density of the commercial electrolyte (1.3634 g/mL for LiPF6 in EC: DMC=1:1 (v:v) electrolyte), the number of LiPF6 salt, EC, and DMC are determined as around 156.52, 1195.63 and 935.63, respectively (see the detailed calculation parameter in Table R1). The numbers we used for LiPF6, EC, and DMC are 156, 1187, and 934 respectively, which results in the error smaller than 1% using this more precise estimation." (2) There is no description to explain how the -1 V vs. PZC was simulated ( Figure S10.3).
Response: We thank the Reviewer for this comment. To address this, we add the following simulation details below in the revised methods (lines 21-29, Page 2 in the revised SI).
"In EC-based electrolyte, the negative charge (ΔQ=0.0036 e) is equally added to each carbon atom in one of the electrodes to generate the polarization field, meanwhile, the other electrode was positively charged by the same amount of charges. According to the parallel plate capacitance model, the voltage can be estimated by the following equation,

= =
Where d is the distance between two electrodes, k is the dielectric constant of electrolyte, A is the averaged area of each carbon atom occupying in a unit cell and is the permittivity of vacuum. The dielectric constant of ECand DX-based electrolytes are around ~25 3 and ~2.2 4 , respectively. Therefore, to simulate a voltage bias value of V ≈ -1 V on the carbon electrodes, we added ΔQ=0.0036 e negative charges to each carbon on the electrode for EC-based electrolyte system, and ΔQ=0.00032 e negative charges to each carbon on the electrode for DX-based electrolyte system." (3) Are 3D periodic boundary conditions applied? According to Figure S10, it appears that a vacuum space is added to separate the two electrodes. If the simulation uses 3D periodic boundary conditions, the 5 nm vacuum space is too short for separating two oppositely charged electrodes. It is recommended to use the vacuum length of twice the electrolyte box to validate your results again.
Responses: We thank Reviewer for this useful comment that allows us to further clarify the MD simulation parameter selection.
First, we confirm that 3D periodic boundary conditions are applied with appropriate line added in the SI (line 42, 43 page 2), As required by Reviewer, all simulation results were verified again using a larger vacuum length (10 nm) as shown in Figures R1a-b. The RDF results confirm that the solvation structure and coordination numbers inside the EDL and bulk electrolyte for EC-based electrolyte (Figures R1c-d) and DXbased electrolyte (Figures R1e-f) are both similar to the results calculated using the 5 nm vacuum distance. It is also worth noting that the coordination number of EC in the bulk electrolyte slightly decreased to about 3 after the vacuum length is increased, which is more consistent with previous reports 5, 6 . Importantly, according to the RDF results, the EC/DMC solvents and FSIanion are still dominating in the first solvation shell for EC-based and DX-based electrolytes, respectively. Besides, Figures R1g and R1h show that the distributions of cation, anion and solvent along the out-of-plane direction follow the same trend as the results in Figure 4 in the original main manuscript.
To address this comment, we add the details of new simulation box in Figure S10.1 of SI and amended simulation results in the revised manuscript.  Nevertheless, according to the results of this study, and supported by the additional experiments described below and included in the SI, we believe that the competitive coordination effect of solvent and anion toward lithium-ion, which determines the lithium-ion solvation structures, is more important than the sole effect of different anion (PF6 and FSI) chemistries in terms of solvent or anion preferential decomposition 7,8 . The SEI formation in EC/DMC with different anions (PF6/FSI) were previously discussed in the supplementary information ( Figure S8.1) of the original manuscript. The effective modulus and viscosity measurements in Figure S8. "The above considerations suggest (and are supported by the explanations below based on the controlled experiments) that the competitive coordination effect of solvent and anion toward cations, which determines the lithium-ion solvation structures, is more important than the sole effect of different anion (PF6 and FSI) chemistries in terms of electrolyte decomposition paths (solvent or anion dominated).
This can be further explained by the Raman spectra of LiFSI in EC/DMC as shown in Figure R2a. The strong EC-Li coordination peaks, at around 723 cm -1 and 915 cm -1 , are observed in LiFSI in EC/DMC electrolytes at concentrations ranging from 0.1 to 6 M, indicating the strong coordination ability of high dielectric constant EC solvent compared with FSI anions 9 . This solvent preferential coordination is similar to the 1M LiPF6 in EC/DMC as proved in the Raman spectrum in Figure 5b of the main manuscript. Additionally, comparing Figure R2b with R2c, the S-F-S stretch peak derived from the aggregated Li-FSI coordination at around 747 cm -1 does not exist in EC/DMC based electrolyte, confirming that the lithium ions in 1M LiFSI mixed with EC/DMC electrolyte mainly form uncoordinated free ion and separated ion-pairs, rather than the ion aggregates as in weakly solvating DXbased electrolyte. In conclusion, Li-ions in 1M LiFSI in EC/DMC are mainly solvated by solvent molecules in the first solvation shell, similar to 1M LiPF6 in EC/DMC electrolyte, but different from 1M LiFSI in DX.
As a result, the SEI measurement by AFM ( Figure R1c) found that, similar to the SEI structures formed in 1M LiPF6 in EC/DMC in Figure 3i of main manuscript, a soft organic-rich SEI layer in the sample section and stiff inorganic-rich SEI layer on the basal plane are also formed in the sample cycled in 1M LiFSI in EC/DMC. We, therefore, suggested that it is the solvation structure (competition effects of solvents and anions), rather than the anion species, that dominates the preferential electrolyte decomposition process in these electrolyte systems 10,11 . More significantly, another proof of the importance of solvation structure is as follows: as shown in Figure R2e, in a relatively high concentration (6M) LiFSI in EC/DMC electrolyte, the SEI layer becomes a uniform stiff layer on both sample section and basal plane. This is because when the concentration reaches 6M (LiFSI: EC: DMC≈1:1.3:0.9), almost every oxygen atom in EC is coordinating with Li-ions according to the Raman spectrum in Figure R2a, but these EC is not enough to "surround" all Li-ions and screen the electrostatic charges. In this case, FSI anions inevitably participate in the first solvation shell. This increased the possibility of anion reduction on the electrode surface and results in more anionderived SEI interfacial chemistries, similar to the weakly solvating electrolyte using DX solvent.

LiPF6/LiFSI salt in the non-polar DX solvent system:
To prepare the electrolyte with weak cation-solvent interaction, we intentionally chose LiFSI to mix with DX, rather than LiPF6. This is because that LiPF6 is barely dissolved by most non-polar solvents, such as benzene and DX, but FSI can be dissolved due to its entropy effect induced by large size 12 . These nonpolar, low-dielectric constant, and low-dissociation capability solvents were carefully selected in other recent studies 13,14 to explore the effect of solvation structure (anion/solvent coordination abilities) and have been demonstrated their effectiveness on SEI interfacial chemistry modifications.
Overall, we believed that the competition between the anion and solvents in the first solvation shell can be tailored by changing the strongly/weakly solvating solvent species and salt concentration, which plays a more important role in determining the SEI formations compared with anion chemistries. By modulating the dominating coordinators with lithium-ions, one can control the SEI interfacial chemistries by guiding the electrolyte decomposition paths."